1. Technical Field
This invention relates generally to armors. More particularly, it relates to the use of friction materials as armor.
2. Related Art
Ballistic armor is used in many forms and many applications, including both structural and non-structural applications, for protecting all manner of items from damage due to impact from all manner of ballistic projectiles. The applications include buildings and other structures, all manner of combat and non-combat vehicles, personnel and other applications. For example, historically, combat and non-combat structures and vehicles were protected by heavy metallic armors made from, for example, iron or high alloy steels. As more powerful and sophisticated armor-piercing projectiles were developed, armors made from these conventional materials had to be made more resistant to penetration. This was generally achieved by making the armor thicker and more resistant to impact and penetration, which generally had the disadvantage of making the armor heavier. Examples of existing armor types may be found in various military specifications, such as those which exist for cold-rolled iron and steel, wrought and other types of armor in varying thicknesses.
In response to the development of sophisticated armor-piercing ballistic projectiles and the need for armor which could be used in applications requiring reduced weight, such as various types of aircraft, stronger but lighter types of armor materials have been developed and used. For example, Ti-6Al-4V (nominally 6 weight percent aluminum, 4 weight percent vanadium, balance essentially titanium) combines good penetration resistance and lower density than iron-based armors and, therefore, has been widely used as an armor material. This alloy, which is relatively lightweight, absorbs the energy of a projectile by spreading the energy out across its mass, thereby blunting the tip of the projectile and resisting penetration. As an example, US military specification MIL-DTL-46077F NOT 1 sets forth the military requirements for titanium alloy armor. Various improvements to and modifications of the composition and metallurgical properties and morphology of titanium-based armors have been proposed.
Relatively recently, conventional armors and lightweight armors, including titanium-based armors, have been thwarted by advanced armor-piercing rounds designed to concentrate their energy within a very small area that may melt the armor material. In response, high temperature ceramic-based armors have been developed. Ceramics are used in the fabrication of armors because they typically have high melting points and good high temperature strength and toughness, as well as being relatively lightweight and extremely hard materials. As an example, US military specification MIL-P-46199P NOT 1 specifies the requirements for alumina plate armor. One of the limitations of ceramic armors, however, is that they dissipate the energy of the projectile partially by cracking. Therefore, ceramic armors lack repeat hit capability, i.e., they will not resist penetration if hit in the same position multiple times, and they disintegrate if struck by multiple rounds. Attempts have been made to address this problem, one of which is the use of metal-ceramic laminate or composite armors that have a metal layer or matrix, such as a Ti-6Al-4V layer surrounding a ceramic-based core. Nevertheless, while such materials can provide somewhat improved properties and performance, the ceramic portion eventually cracks in response to multiple projectile impacts, thereby greatly reducing or eliminating the effectiveness of the armor. Moreover, the costs of ceramic and metal-ceramic armors is generally significantly higher than those of other types of armor.
Another type of armor is typically known as reactive armor. Here, the armor includes an ablative or explosive material that reacts by ablation or even explosion when impacted by a ballistic projectile, typically so as to alter the flight of the projectile and its impact zone, thereby providing protection to the item with which it is associated. In explosive reactive armors, the outward force of the reactive armor explosion counteracts the force of the incoming projectile, thereby resisting penetration of the armor. Reactive armor designs may also include movable members that may, for example, absorb the energy of the projectile, blunt the projectile, modify the trajectory of the projectile, and/or destroy the projectile. Reactive armors, however, like ceramic armors, are somewhat deficient in that they do not provide good protection against multiple impacts in the same location. Once the reactive armor is activated, a second round hitting the armor in the same location is much more likely to penetrate the armor or otherwise damage the item being protected.
Various polymers and polymer composites have also been proposed for use as ballistic armor, such as the composite material described in U.S. Pat. No. 7,037,865, which employs the use of a matrix material such as a resin which is filled with various densely packed small particles, such as hollow microspheres, and may also include fibers, as a partial substitute for the particles or the matrix, or a flanking material for the matrix/particle composite.
Numerous types of fabrics, including woven and non-woven fabrics, as well as those which are layered in various combinations, or impregnated with various resins and other materials, or both, have also been employed as ballistic armor for personal protection applications, or body armor, including various forms of garments and head protection articles. These armors are made from polymer fibers, such as various aramid, ultra-high molecular weight polyethylene, polybenzoxazole and other fibers. Such “soft armor” garments and other articles have also been designed to incorporate spaces for the insertion of traditional “hard armor” plate inserts to enhance their resistance to and protection from ballistic projectiles. Since soft armor is frequently used for personal protection, the weight of the armor is very important, and it is desirable to maximize the ballistic resistance and protection while minimizing the weight. Since hard armor inserts can constitute a significant portion of the weight of such soft armor, it is very desirable to identify hard armor suitable for use as inserts that have reduced density and consequently weight as compared to traditional types of armor and which offer equivalent or improved ballistic resistance and protection performance. Body armor is categorized based on its ability to resist penetration by various small caliber projectiles into four subcategories (I-IV) by the National Institute of Justice under NIJ Standard 0101.4. Various US military specifications have also been developed for “soft” body armor and “hard” body armor inserts and define the operational and performance requirements for these materials
Despite the many existing forms of armor described above, there remains need for new lightweight armor materials for various armor applications, particularly those which have multi-shot capability (resistance to multiple impacts) and reduced density and consequently weight as compared to existing types of armor and which offer equivalent or improved ballistic projectile resistance and protection performance.